The present invention relates to a nuclear fuel assembly for a pressurized water reactor, and in particular to a nuclear fuel rod spacer or spacer grids which are placed at predetermined distances along the height of the fuel assembly in order to provide lateral bracing and spacing, and to maintain the nuclear fuel rods in fixed positions.
In a nuclear reactor, the reactor core contains nuclear fuel which is typically in the form of fuel rods grouped together in fuel assemblies. Groups of fuel assemblies are arranged into a matrix to form a core capable of controlled fission reaction.
Each fuel rod is typically a long member approximately 0.4 inches in diameter and 8 to 15 feet long containing fuel usually in the form of a stack of fuel pellets which are surrounded by tubular cladding. The fuel rods which make up an assembly are grouped together to form a plurality of longitudinally extending members which are supported by two parallel end plates, an upper and a lower tie plate. These plates are usually connected to one another by guide tubes or tie rods or other structural elements.
Each fuel assembly or bundle may also include nonfuel bearing members. Examples include guide tubes to form passageways for control rods which assist in controlling the rate of fission, instrumentation tubes for in-core instrumentation, spacer capture rods, and water rods to modify the neutron moderation in the assembly. The spaces between adjacent fuel rods create flow channels through which coolant and/or moderator can circulate. In light water reactors, the coolant and moderator is water. Lateral bracing and spacing of the fuel rods in the fuel assembly are provided by spacers or spacer grids. A typical fuel assembly for pressurized water reactor is described more fully in U.S. Pat. No. 5,787,142 which is hereby incorporated by reference.
The fuel assembly or bundle, whether in a pressurized water reactor, boiling water reactor, high temperature gas cooled reactor, or any other type of reactor, functions in part to maintain the fuel rods in a fixed position, ideally free of vibration and restrained from bowing or other lateral displacement during normal and other operating conditions. In addition, by maintaining the fuel rods in fixed positions, proper cooling and neutron moderation can be achieved. Devices that assist in maintaining the fuel rods in fixed positions in the fuel assembly or bundle and which thereby facilitate proper fuel cooling are spacers.
Spacers or spacer grids which provide lateral bracing are typically designed to allow differential axial expansion of the fuel rods. Springs incorporated in some spacer grids are most frequently used to permit some sliding of the fuel rods with respect to the spacer grids. In some of the designs, the spacer grid is free to move axially a small amount to accommodate minor changes in the axial length of the fuel rods during irradiation.
If spacers were to be rigidly connected to the fuel rods as well as to structural members of the fuel assembly, then relative axial movement due to rod growth and thermal expansion of adjacent rods could cause local fuel rod skewing and bowing.
By being positioned at regular intervals, spacers maintain rod-to-rod spacing along the length of the fuel assembly. Spacers are typically made of zirconium based alloy sheet material or sometimes from Inconel or stainless steel, and are built up from a relatively large number of different intricately shaped strips.
These and other objects of the invention are achieved by providing a spacer for nuclear fuel rods of a nuclear fuel assembly, comprising a plurality of sets of two horizontally extending substantially straight parallel grid strips, one of the two horizontally extending substantially straight parallel grid strips being substantially flat and a second one of the two horizontally extending substantially straight parallel grid strips having generally vertically extending undulations along its length forming tubular passages between the two horizontally extending substantially straight parallel grid strips to produce swirling motion of coolant, the plurality of sets of two horizontally extending substantially straight parallel grid strips arranged to intersect one another to form a lattice of polygonal shaped cells through which nuclear fuel rods extend and which support the fuel rods positioned asymmetrically within the cells.
In another embodiment of the present invention a nuclear fuel assembly for pressurized water reactors, the assembly having a plurality of elongated fuel rods supported between a lower tie plate positioned toward the bottom of the assembly and an upper tie plate positioned toward the top of the assembly, at least one spacer for positioning and retaining the fuel rods, the spacer comprising a plurality of sets of two horizontally extending substantially straight parallel grid strips, one of the two horizontally extending substantially straight parallel grid strips being substantially flat and a second one of the two horizontally extending substantially straight parallel grid strips having generally vertically extending undulations along its length forming tubular passages between the two horizontally extending substantially straight parallel grid strips to produce swirling motion of coolant, the plurality of sets of two horizontally extending substantially straight parallel grid strips arranged to intersect one another to form a lattice of polygonal shaped cells through which nuclear fuel rods extend and which support the fuel rods positioned asymmetrically within the cells.